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The History of Evolutionary Thought   

Uniformitarianism: Charles Lyell

gradual change by erosion
Discrete rock layers containing different fossils reinforced the idea that the Earth's history could be divided into ages marked by catastrophic change. However, gradual change, like that caused by erosion, has also played an important role in the Earth's history.

Thanks to the pioneering work of researchers such as William Smith, geologists in the early 1800s were able to swiftly organize rock formations into a single colossal record of Earth's history. Many geologists saw in this record a stormy epic, one in which our planet had been convulsed repeatedly by abrupt changes. Mountains were built in catastrophic instants, and in the process whole groups of animals became extinct and were replaced by new species. Giant tropical plants, for example, left their fossils in northern Europe during the Carboniferous Period, never to be seen there again. Earth's history might not fit a strict Biblical narrative any longer, but these revolutions seemed to be a sign that it did have a direction. From its formation, catastrophes altered the planet’s surface step by step leading towards the present Earth. Life, likewise, had its own arrow through time.

Catastrophism
Even before this geological evidence had emerged, some naturalists had already claimed that Earth's history had a direction. Buffon, and later the physicist Joseph Fourier, both claimed that the Earth had begun as a hot ball of molten rock and had been cooling through time. Fourier argued that the tropical plants of Europe must have lived during those warmer times. Some geologists suggested that the cooling of the planet occasionally triggered violent, sudden uplifts of mountains and volcanic eruptions.

Charles Lyell
"Catastrophism," as this school of thought came to be known, was attacked in 1830 by a British lawyer-turned-geologist named Charles Lyell (1797-1875). Lyell started his career studying under the catastrophist William Buckland at Oxford. But Lyell became disenchanted with Buckland when Buckland tried to link catastrophism to the Bible, looking for evidence that the most recent catastrophe had actually been Noah's flood. Lyell wanted to find a way to make geology a true science of its own, built on observation and not susceptible to wild speculations or dependent on the supernatural.

The rock cycle
The ideas of Hutton and Lyell led to an understanding of "the rock cycle" as we know it today.
Gradual change
For inspiration, Lyell turned to the fifty-year-old ideas of a Scottish farmer named James Hutton. In the 1790s, Hutton had argued that the Earth was transformed not by unimaginable catastrophes but by imperceptibly slow changes, many of which we can see around us today. Rain erodes mountains, while molten rock pushes up to create new ones. The eroded sediments form into layers of rock, which can later be lifted above sea level, tilted by the force of the uprising rock, and eroded away again. These changes are tiny, but with enough time they could produce vast changes. Hutton therefore argued that the Earth was vastly old — a sort of perpetual-motion machine passing through regular cycles of destruction and rebuilding that made the planet suitable for mankind.

Valley formed by erosion
Lyell found evidence that valleys were formed through the slow process of erosion, not by catastrophic floods.

Lyell traveled through Europe to find more evidence that gradual changes, the same we can see happening today, had produced the features of the Earth's surface. He found evidence for many rises and falls of sea level, and of giant volcanoes built on top of far older rocks. Processes such as earthquakes and eruptions, which had been witnessed by humans, were enough to produce mountain ranges. Valleys were not the work of giant floods but the slow grinding force of wind and water.

Uniform Processes of Change
Lyell's version of geology came to be known as uniformitarianism, because of his fierce insistence that the processes that alter the Earth are uniform through time. Like Hutton, Lyell viewed the history of Earth as being vast and directionless. And the history of life was no different.

Tenerife, Canary Islands

Lyell crafted a powerful lens for viewing the history of the Earth. On Darwin's voyage aboard the Beagle, for example, he was able to decipher the history of the Canary Islands (right) by applying Lyell's ideas to the volcanic rock he encountered there. Today satellite measurements reveal that mountains may rise an inch a year, while radioactive clocks help show how they've been rising that way for millions of years. But Lyell could never have grasped the mechanism — plate tectonics — that makes this kind of geological change happen.

Yet geologists today also know that some of the factors that changed the Earth in the past cannot be seen at work today. For example, the early Earth was pummeled by gigantic hunks of solar debris, some as big as Mars. For the first one or two billion years of Earth's history, plate tectonics didn't even exist as we know it today.

Lyell had an equally profound effect on our understanding of life's history. He influenced Darwin so deeply that Darwin envisioned evolution as a sort of biological uniformitarianism. Evolution took place from one generation to the next before our very eyes, he argued, but it worked too slowly for us to perceive.



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Biostratigraphy: William Smith

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Discrete Genes Are Inherited: Gregor Mendel


Rock layers image courtesy of David Smith, UCMP; Lyell image courtesy of Dennis O'Neil, Palomar College; Valley image courtesy of David Smith, UCMP; Tenerife, Canary Islands image courtesy of Roberto Bertero.

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